The present invention relates to a technique for detecting a combustion condition of an internal-combustion engine and in particular it relates to a technique for detecting a misfire of the engine.
Recently, in accordance with enforcements of emission regulations for motor vehicles, precision of detecting a misfire of an engine needs to be improved in order to suppress emission of HC and deterioration of catalysts caused by a misfire.
The Japanese Patent Publication No. H11-82150 (Reference 1) discloses a technique for detecting by a pressure sensor disposed in a cylinder of an engine an occurrence of a misfire based on asymmetry of cylinder pressures before and after a top dead center (TDC).
The Japanese Patent Publication No. S64-015937 (Reference 2) discloses a technique for detecting an occurrence of a misfire by comparing an integral value of the values detected by a pressure sensor disposed in a cylinder of an engine with a threshold value.
The Japanese Patent Publication No. S60-166739 (Reference 3) discloses a technique for correcting ignition timing based on a deviation between cylinder pressure detected in accordance with an output of a pressure sensor disposed in a cylinder of an engine and cylinder pressure that is predetermined with respect to a reference combustion condition.
The integral value of the cylinder pressures or the asymmetry of the cylinder pressures which are utilized in the above-described Reference 1 and Reference 2 may vary due to influence of variations in combustion for each combustion cycle. During a low load time, misfire determination may become difficult because a difference between cylinder pressures at a normal operation time and at a misfire is small.
The technique of the above-described Reference 3 utilizes a deviation between a reference cylinder pressure to be predetermined for each operating condition and an actually-measured cylinder pressure. Since peak values of the cylinder pressures may have some dispersion among engines and may change due to certain aging, precision of misfire detection is unreliable.
Even when no combustion occurs in a cylinder of an engine (that is, at a misfire), a pressure in the cylinder increases by a compression movement of a piston, which is referred to as motoring. When the engine is in a low load condition, it becomes difficult to distinguish between a normal combustion and a misfire because a majority of the detected pressures are the pressures caused by motoring. Thus, there is a need for a technique for enabling detection of a misfire in a cylinder even under such low load condition.